A typical orthogonal code table is shown in FIG. 1. A related art orthogonal encoder receives a total of four bits, and the number of cases of bit streams is 16 (=24). Generally, orthogonal codes having 16 codes must be 16 bits in length. An example of Walsh orthogonal codes is illustrated in FIG. 1.
It is important in the orthogonal code that columns of the Walsh codes are orthogonal to each other. For example, a cross-correlation value between the first Walsh code “0000000000000000” and the second Walsh code “0101010101010101” becomes zero, as expressed in the following Equation. This means that two codes are orthogonal to each other.
  c  =                    1        16            ⁢                        ∑                      i            =            0                    15                ⁢                              (                                          2                *                                  CODEld                  i                                            -              1                        )                    ×                      (                                          2                *                CODE                ⁢                                                                  ⁢                2                ⁢                                  d                  i                                            -              1                        )                                =    0  
An auto-correlation value calculated using the first Walsh code becomes “1”, as expressed in the following Equation.
  c  =                    1        16            ⁢                        ∑                      i            =            0                    15                ⁢                              (                                          2                *                                  CODEld                  i                                            -              1                        )                    ×                      (                                          2                *                                  CODEld                  i                                            -              1                        )                                =    1  
Since a related art orthogonal coding scheme needs 16 code bits in order to transmit a total of 4 data bits, its spectrum efficiency is 0.25. Generally, an orthogonal modulation system exhibits good performance in a situation where interference is generated by a diffusion effect. Such a scheme is used in Interim Standard (IS)-95, which is one of cellular communication standards.
However, such a system has very low spectrum efficiency, causing channel waste. Therefore, there is a need to develop a new orthogonal code modulation scheme that has high spectrum efficiency and is robust against interference.